Cell Bio Cell Signaling

Cell Signaling

Cells receive (from other cells and from their environment), send and interpret a variety of signals
-Signals/information comes in variety of forms and often have to be converted from one form to another
-Signaling cell produces a particular signal molecu

Signal transduction

process of signal conversion

4 Types of signaling

*Endocrine
*Paracrine
*Neuronal
*Contact-dependent

Endocrine

signaling cell secretes a signal that travel throughout the body (via blood stream) and eventually binds to a receptor on the target cell ==> long-range signaling

Paracrine

signaling cell secretes a signal that travels locally and binds to a receptor on a nearby target cell ==> local signaling

Neuronal

neuron sends an electrical signal along its membrane which is transduced into a chemical signal where it contacts (synapse) the target cell ==> very rapid local or long-range signaling

Contact-dependent

a protein on the surface of the signaling cell binds to a receptor protein on the surface of the target cell ==> intimate local signaling

Whether or not a cell responds to a signal and how it responds depends

on the nature of its signal receptors

Receptors

-The type and number of receptors varies from cell type to cell type and over time
-Different cells may respond differently to the same signal molecule

Two types of signal receptors on target cells

*Cell surface receptors - usually transmembrane proteins
*Intracellular receptors - soluble cytosolic or nuclear proteins

Intracellular signaling pathways

After binding of signaling molecule to its receptor, the message is passed from one intracellular signaling molecule to another until signal reaches its final target inside the cell

signaling cascades

Receptors relay signals via pathways of molecular interactions within cells

Signaling cascades can:

� transfer signals
� transform signals
� amplify signals
� distribute signals
� modulate signals

Intracellular Receptors

Intracellular receptors are cytosolic or nuclear molecules that bind to chemical signals (small hydrophobic molecules) that can cross the plasma membrane

Examples of signals that bind to steroid hormone receptors:

*steroid hormones
� cortisol
� estradiol
� testosterone
*thyroid hormones
*vitamin D
*retinoids

Steroid receptor proteins are

Steroid receptor proteins are DNA-binding proteins that regulate genes
ligand-binding changes the configuration of the receptor, altering its ability to
bind to DNA, thereby activating or inhibiting transcription

Example of other molecules acting through intracellular receptors

Nitric oxide (NO)

What happens if a molecule is too large or hydrophilic to cross the plasma membrane?

Majority of signaling molecules (proteins, peptides, water-soluble molecules) are too large or hydrophilic to cross the plasma membrane
-These signaling molecules bind to receptors that are transmembrane proteins
Transmembrane receptors receive a signal o

Intracellular signaling proteins can function as molecular switches:

*signal molecules cycle from active to inactive state
*signal can be passed from one molecule to another giving rise to a cascade of signaling events

Signaling by phosphorylation

� signal protein activated by phosphorylation
� mediated by a protein kinase
� requires ATP
� signal protein deactivated by dephosphorylation mediated by a protein phosphatase

G-proteins

� G-protein activated by binding of GTP
� G-protein inactivated by hydrolysis of GTP

Three classes of receptors:

*Ion-channel-linked receptors
*G protein-linked receptors
*Enzyme-linked receptors
For many signaling molecules there is more than one type of extracellular receptor

Ion-channel-linked receptors or transmitter-gated ion channels

Mainly responsible for the rapid transmission of signals across synapses in the nervous system and in the electrically excitable cells such as muscle
� signal binding opens a transmembrane channel
� change in intracellular ion concentration or membrane po

G protein-linked receptors

� signal binding stimulates cytosolic part of receptor to bind a G protein ==> activates G protein
� the G protein binds to one or more effector proteins and initiates a signaling cascade
- Largest family of cell surface receptors; mediate response to a l

Receptor - single polypeptide, seven-pass transmembrane protein:

� extracellular ligand-binding domain
� internal G-protein binding domain
� activation of the receptor by a ligand leads to activation of a G protein

G-proteins composed of three subunits (trimeric protein)

*alpha subunit
� binds GTP
� hydrolyzes GTP to GDP (GTPase)
*beta subunit + gamma subunit
� dimer that binds to the cytosolic face of the membrane

Upon activation or G-protiens:

GDP dissociates from the alpha subunit ==> GTP binds to the alpha subunit ==> alpha subunit dissociates from the beta-gamma dimer ==> alpha subunit binds to a target protein and initiates a response
After a few seconds the alpha subunit hydrolyzes the GTP

Types of G-proteins

About 20 different types of G-proteins identified in mammals

Two broad classes of action:

*Regulation of ion channels
� Ca++ channels
� K+ channels
*Regulation of membrane-bound enzymes ==> small molecule second messengers:
� adenylate cyclase ==> cAMP
� phospholipase C ==> DAG & IP3

Cyclic AMP Pathway

Regulation of membrane-bound enzymes

Cyclic AMP

Cyclic AMP is a common second messenger produced by the enzyme adenylate cyclase, the most frequent target enzyme for G proteins in cells

Adenylate cyclase

� activated by stimulatory G proteins (Gs)
� inhibited by inhibitory G proteins (Gi)

Cyclic AMP destroyed by

cAMP phosphodiesterase
� hydrolyzes cAMP to 5'-AMP

Many cell responses to signals are mediated by cAMP

*cAMP concentration in cells can change rapidly in response to extracellular signals
*cAMP is water soluble and diffuses from the vicinity of the membrane where it is made to target proteins in cytosol or organelles
*cAMP works mainly by binding to the re

Effects of cAMP vary between different cell types

In different cell types, different sets of target proteins are phosphorylated by PKA
Examples
Adrenaline signaling via G-protein-linked receptor (prepares body for action)
*adrenaline-induced activation of glycogen breakdown in skeletal muscles ==> glucos

Phospholipase C-mediated Signaling

Regulation of membrane-bound enzymes

Phospholipase C

membrane-bound enzyme activated by G-protein
-when activated by G-protein, cleaves the plasma membrane glycolipid, phosphatidylinositol 4,5-bisphosphate (PIP2) into two second messengers

Two second messengers in phospholipase C

*diacyl glycerol (DAG)
� plasma membrane lipid
� associated with the cytosolic face of the membrane
� activates the enzyme, protein kinase C (PKC) which phosphorylates other molecules ==> triggers kinase cascade
*inositol 1,4,5-trisphosphate (IP3)
� solub

Calcium as an Intracellular Signal

Ca++ is a common and very important intracellular messenger triggered by variety of signals
Calcium Concentration:
Cytosolic Ca++ concentration - very low = 10-7 M

How is calcium kept at a low concentration?

*ATP-driven membrane pumps
� pump Ca++ out of the cell
� pump Ca++ into ER and mitochondria
*Na+-Ca++ antiport
� pumps Ca++ out of the cell

Effects of Ca++ in the cytosol are mediated by

the interaction of Ca++ with a variety of Ca++-binding proteins

Calmodulin

common calcium-binding protein that affects the activity of many other proteins
Ca++ binding to calmodulin ==> conformational change in calmodulin molecule ==> calmodulin binds to (wraps around) target molecule ==> altered protein activity

Ca++/ Calmodulin-dependent Protein Kinases (Ca-M Kinases)

� very important calmodulin targets
� mediate cellular responses to Ca++
� phosphorylate other proteins on serine and threonine side chains ==> activation of the target protein

Enzyme-linked receptors

� single pass alpha helical transmembrane proteins
� cytoplasmic domain acts as an enzyme or interacts with another protein that acts as an enzyme
� signal binding activates a catalytic domain in the cytosolic part of the receptor, the catalytic domain is

Examples of Enzyme-linked receptors

*Guanyl cyclase receptors
� ligand-binding generates cyclic GMP (cGMP)
*Receptor tyrosine kinases
� the largest class of enzyme linked receptors
� cytoplasmic domain functions as a tyrosine protein kinase (phosphorylates tyrosine side chains on selected i

Receptor tyrosine kinases signaling pathway:

� ligand-binding causes two receptors to become crosslinked into a dimer
� dimerization activates the kinase activity of the cytosolic domain of the two proteins
� each protein phosphorylates the other (autophosphorylation)
� various cytosolic signal mole

Growth factors

proteins that control cell proliferation

Main subfamilies of growth factor receptor tyrosine kinases

� epidermal growth factor (EGF)
� insulin (IGF)
� nerve growth factor receptor (NGF)
� platelet-derived growth factor (PDGF)
� fibroblast growth factor (FGF)
� vascular endothelial growth factor (VEGF)

Describe Ras protein

an important GTP-binding protein that acts as an intracellular signal (virtually all receptor protein kinases activate Ras)
� is associated with the cytosolic face of the plasma membrane
� is activated by binding to an adaptor protein that binds to phosph

Ras relays signals from receptor tyrosine kinases to the cell nucleus to

control transcription and cell replication

Ras gene is often mutated in cancer cells, leading

Ras gene is often mutated in cancer cells, leading cells to uncontrolled proliferation (~30% of human cancers have mutations in ras genes ==> Ras activity permanently switched on; even in absence of proliferation-stimulating extracellular signals)

Ras relays signals from receptor tyrosine kinases to the cell nucleus by

Ras relays signals from receptor tyrosine kinases to the cell nucleus by promoting activation of a phosphorylation cascade ==> series of protein kinases phosphorylate and activate one another in sequence

Not all enzyme-linked receptors trigger complex signaling cascades

some receptors use more direct route to control gene expression (e.g. interferon receptor)

Intracellular Signaling by Phosphorylation Cascades

Many cell surface signals are transduced by a series of protein phosphorylation reactions, mediated by protein kinases
� Serine/threonine kinases
phosphorylate proteins on serine and threonine side chains
� Tyrosine kinases
phosphorylate proteins on tyros

Phosphorylation

protein activation
Kinases often phosphorylate and activate other kinases in a sequential cascade ==> phosphorylation of downstream signal molecules

The ultimate targets of the cascade are:

� one or more nuclear proteins that affect gene expression
� cytosolic proteins that affect cell functions

Extra notes about protein kinases

Most of signaling pathways eventually activate protein kinases
Protein kinases from different signaling cascades often interact and regulate components of other signaling pathways
Complex networks of signaling pathways coordinate and integrate cell behavi

Intracellular Signaling - Integration of information

Cell receives signals from many sources and must integrate them to generate an appropriate response
Cross talk between signaling pathways cells are able to react to the combination of signals
Some intracellular signaling proteins act as integrating device